Heating-load estimate by area
Multiply heated floor area by a specific heat-loss input to compare preliminary heating-power scenarios. The W/m² input must come from a justified assessment of the building. This tool does not calculate a room-by-room design heating load from climate, ventilation and envelope data.
Reviewed:
Calculator
Area method
Q [kW] = Aheated [m²] × q [W/m²] / 1000
The output is proportional to both entered values. Double the area at the same q and the estimate doubles. q must reflect the building and design conditions; a universal age-based default would hide important differences. Heated floor area is not external wall area. Do not add an arbitrary sizing margin to an already conservative design input.
How much heating power for 50 m²?
For 50 m² and an illustrative q = 50 W/m², the estimate is 2.5 kW. At 30 W/m² it is 1.5 kW; at 80 W/m² it is 4.0 kW. These scenarios demonstrate sensitivity, not recommended values for an unidentified building. The same 50 m² can need different power when climate, airtightness or glazing changes.
Before equipment sizing
A final heat-load assessment needs design outdoor and indoor temperatures, transmission losses, thermal bridges, ventilation and infiltration, room boundaries and any justified reheating allowance. Equipment also needs compatible operating temperatures, minimum modulation and system design. This area estimator cannot select a boiler, heat pump or radiator or certify a standard calculation. Keep the assessment source beside the entered W/m².
Where q comes from
q [W/m²] = Qdesign [kW] × 1000 / Aheated [m²]
Use a documented heat-load assessment for the same heated area, indoor/outdoor design temperatures and ventilation conditions. For example, an existing assessment of 6 kW for 100 m² corresponds to 60 W/m². This division summarizes an existing result; it does not calculate a new design load. Do not transfer that value to another building without assessment.
The DOE source below explains why equipment sizing starts with a building load calculation and why rules of thumb can oversize equipment. Its U.S. sizing context does not supply a universal q or certify compliance with European standards. NIST supports the unit conversions, not the choice of heat load.
Heating estimate for 100 m²
For 100 m², illustrative inputs 30, 50 and 80 W/m² give 3, 5 and 8 kW. Area alone is insufficient. Enter a justified specific heat loss in the calculator and retain its source. These scenarios are not equipment sizing or a room-by-room design heat-load assessment.
Heating estimate for 150 m²
For 150 m², illustrative inputs 30, 50 and 80 W/m² give 4.5, 7.5 and 12 kW. Area alone is insufficient. Enter a justified specific heat loss in the calculator and retain its source. These scenarios are not equipment sizing or a room-by-room design heat-load assessment.
Watts to BTU per hour
1 W = 3.412141633 BTU/h
1000 W is 3412.142 BTU/h; 12000 BTU/h is about 3516.853 W. BTU/h is power, while BTU is energy. Multiply by hours to obtain energy; this conversion alone does not size heating or cooling equipment.
Calculator — Watts to BTU per hour
Heat loss through one element
Q [W] = U × A × ΔT
20 m² at U = 0.25 and ΔT = 30 K gives 150 W or 511.821 BTU/h. A 30 °C temperature difference equals 30 K. This steady-state transmission term excludes ventilation, thermal bridges and reheating; it is not the total building heating load.
Calculator — Heat loss through one element
Room dimensions and supplied W/m²
Q [W] = length × width × q; BTU/h = Q × 3.412141633
5 m × 4 m at an illustrative 70 W/m² gives 1.4 kW and 4777 BTU/h. Supply a justified q; dimensions alone cannot determine demand. This preserves the simple area estimator and does not replace a room-by-room assessment.
Calculator — Room dimensions and supplied W/m²
Worked examples — illustrative inputs
| A [m²] | q [W/m²] | Q [kW] |
|---|---|---|
| 50 | 30 | 1.50 |
| 50 | 50 | 2.50 |
| 50 | 80 | 4.00 |